In order to prevent decay of wood and timbers, and thereby increase their life, it is common practice to impregnate the wood or timbers with a preservative such as creosote, mixtures of inorganic compounds which are dissolved or dispersed in water, or certain organic compounds which are dissolved in organic solvents. The protection afforded by the application of these materials is dependent upon deep and reasonably uniform penetration into the wood or timber by the preservative material.
The subject of wood treatment and wood preservation is discussed in some detail in the two-volume treatise entitled "Wood Deterioration and its Prevention by Preservative Treatments", Darrel D. Nicholas, Editor, Syracuse Wood Science Series 5, Syracuse University Press, Syracuse, N.Y., 1973. Among the examples of wood preservatives described therein are various creosote compositions, pentachlorophenol, copper naphthenate, copper-8-quinolinolate, organotin compounds, organomercury compounds, zinc naphthenate, chlorinated hydrocarbons, ammoniacal copper arsenate (ACA), acid copper chromate (ACC), zinc salts such as zinc chloride, zinc oxide and zinc sulfate, chromated copper arsenate (CCA), etc.
Wood preservatives such as those described above have been applied to the wood as solutions, emulsions, pastes or dispersions in liquid hydrocarbons and/or aqueous systems. In many applications, the use of aqueous systems is preferred over liquid hydrocarbons because of the odors, flammability and often toxic nature of the liquid hydrocarbons. U.S. Pat. No. 4,507,152 describes aqueous compositions having fungicidal and insecticidal properties which can be used in the treatment of wood. The aqueous compositions comprise oil-soluble metal salts of organic carboxylic acids, halopyridyl phosphates and surfactants. The compositions can be utilized to penetrate wood, and the wood treated with this aqueous system is resistant to fungi and insects.
Although a number of relatively non-toxic aqueous systems have been suggested for preserving wood, many of the wood treating systems used commercially utilize solutions of oil- or hydrocarbon-soluble preservatives. For example, the American Wood-Preservers' Association Standard P9-87 entitled "Standards for Solvents and Formulations for Organic Preservative Systems" describes five hydrocarbon solvent types for preparing solutions of preservatives such as pentachlorophenol, copper naphthenate, etc. The Type A solvent is composed of petroleum distillates or a blend of petroleum distillates and co-solvents provided that the blended solvent meets certain specifications (for example, in practice, mixtures containing high quantities of aromatics are often use with pentachlorophenol to provide pentachlorophenol solvency without the addition of a co-solvent); Type B solvent is based on a volatile petroleum solvent (LPG); Type C solvent is a light hydrocarbon solvent with auxiliary solvent; Type D solvent is a chlorinated hydrocarbon solvent-inhibited grade of methylene chloride; and Type E solvent is an organic solvent composed of petroleum distillates or a blend of petroleum distillates and co-solvents for preparing solutions of pentachlorophenol and dispersions of these in water.
U.S. Pat. No. 4,374,852 describes anti-fungal compositions comprising zinc or copper salts of organic acids which are useful as wood preservatives. Copper and zinc salts are most commonly dissolved in organic solvents such as petroleum- and co-derived solvents such as white spirit, paraffin, gas oil, xylene or naphtha. In U.S. Pat. No. 3,785,770, a wood preservative composition is described which comprises solutions of pentachlorophenol in mineral spirit solvents and, optionally, co-solvents which may be xylene or cyclohexanone.
In some of the applications, the solvents are halogenated hydrocarbon solvents which are relatively low boiling (e.g., less than 140.degree. C. and generally less than 100.degree. C.) such as carbon tetrachloride, chloroform, etc. Such halogenated solvent systems are preferred in wood treatments where it is desirable to remove the solvent after the preservative has penetrated into the wood. U.S. Pat. Nos. 4,013,804 and 3,874,908 are examples of patents describing wood preservative systems containing low boiling solvents. U.S. Pat. No. 3,874,908 describes a process for impregnating wood which utilizes a solution or dispersion of a halogenated hydrocarbon solvent, a wood preservative, and an anti-blooming additive which may be ethylene glycol, propylene glycol, liquid polyglycols of molecular weights of up to about 4000, or lower alkyl monoethers thereof. Suitable halogenated hydrocarbon solvents are described as those which have boiling points of from about room temperature up to about 140.degree. C., preferably up to about 100.degree. C.
The most common commercial procedure for impregnating wood involves contacting the wood with the preservative under relatively high pressure such as 50-150 pounds per square inch for a substantial period of time such as from one hour to 24 hours. The process also may require relatively high temperatures such as about 75.degree. C. to about 105.degree.-110.degree. C.
U.S. Pat. No. 3,200,003 describes a process for impregnating wood with preservatives such as pentachlorophenol and copper guinolinolate which utilizes a solution of the preservative in an aliphatic hydrocarbon solvent which boils below the boiling point of water at ambient atmospheric pressure and readily liquefies at ambient atmospheric temperatures, and a co-solvent such as toluene, benzene, nitrobenzene, isopropyl ether, etc. The process is illustrated on Southern Yellow Pine, Douglas Fir and Red Elm Lumber.
U.S. Pat. No. 4,051,282 describes the production of treated wood with improved penetrability by projectiles. The treating solution utilized in the process contains an impregnant, an aliphatic hydrocarbon or halogenated hydrocarbon liquid carrier, a co-solvent if the solubility of the impregnant in the liquid carrier is inadequate, and an effective amount of lubricating oil. After the treating solution has impregnated the wood, the liquid carrier is evaporated leaving wood containing an impregnant with internal lubrication due to the oil and possibly some co-solvent. The amount of oil used in the process is an effective lubricating amount. The patentees report that the particular effective amount of oil will vary for different species of wood, and for Douglas Fir and Southern Yellow Pine, the practical effective amount of oil added to the treating solution is in the range of around 1.5% to around 15% of the total treating solution (col. 4, lines 13-17). The patentees also disclose that the aliphatic hydrocarbon or halogenated hydrocarbon carriers have boiling points above about 35.degree. C. and below about 130.degree. C.
In order for the chemical treatment to be effective in preserving wood, it is desirable that there be adequate retention of the preservative in the wood and that there is a deep impregnation of the chemicals, particularly the preservatives, into the wood. The extent of penetration and retention obtained by any given process will depend upon the nature of the preservative, the operating conditions, the nature of the wood, etc. For example, it is particularly difficult to penetrate certain types of wood poles or logs such as those derived from Douglas fir, western hemlock, hemfir, etc. It is generally desirable that the preservative penetrate into the wood and extend at least throughout all of the sapwood, and more preferably into the heartwood.
The above-described prior art represents a small sampling of the suggestions which have been made for treating wood with preservative materials to prevent decay. In spite of these many suggestions made in the prior art, there continues to be a need for inexpensive and safe treatments which are effective and which result in deeper and more uniform penetration of the preservative and other chemicals to the core of the wood, especially when the wood to be treated is difficult to penetrate. Difficult-to-penetrate woods are often referred to in the art as "refractory".